1. Field of the Invention
The present invention relates to a laser light generating method and apparatus for generating laser light by locking the phases of a plurality of laser resonators to each other.
2. Description of the Related Art
The injection locking chain (refer to E. A. P. Cheng and T. J. Kane, Opt. Lett. 16, pp. 478-480 (1991)) is known as an example of laser light generating methods in which a plurality of laser resonators resonate simultaneously, that is, methods for generating laser light by locking the phases of a plurality of laser resonators to each other.
The injection locking chain will be described below with reference to FIG. 1.
In the injection locking method, a small-size laser is used as a master laser 1 and laser light emitted from the master laser 1 is guided by a mirror 2, an electro-optic modulator 3, and a mirror 4 to a large-size slave laser (slave resonator) 22 that is composed of a precision positioning mirror 10 as a resonator optical path length control means and mirrors 5, 7, and 9, whereby the laser light is amplified at a large gain without losing coherence.
In doing frequency locking in the slave resonator 22, locking should be made to the frequency of the master laser light by precisely controlling the resonator optical path length of the slave resonator 22. That is, the frequency in the resonator 22 is locked by a control loop that consists of a frequency supplying circuit (phase modulation signal oscillation circuit) 30 for supplying a phase modulation signal to the electro-optic modulator 3, a photodetector 32 for detecting, as a phase detection signal, part of laser light exiting from the resonator 22, and a mixer 31 for supplying a control signal (error signal or positioning signal) to the precision positioning mirror 10 by synchronously detecting the phase detection signal and the phase modulation signal.
As described above, the FM side band method is used as a frequency locking method. That is, the electro-optic modulator 3 as a phase modulation element is disposed in the optical path before the slave resonator 22 and frequency locking is performed by feeding back, as an error signal, a signal obtained by synchronously detecting the phase modulation signal for the electro-optic modulator 3 and return light from the slave resonator 22 to the precision positioning element 10 that is disposed in the resonator 22.
The injection locking chain is a method for increasing the laser light output power by arranging, in series, a plurality of such slave resonators (slave resonators 22 and 23) that operate according to the injection locking method. In this case, each slave resonator has an independent control loop according to the FM side band method and each electro-optic modulator is disposed on the optical path before the associated slave resonator. In the injection locking chain, it is necessary to cause the plurality of laser resonators to resonate simultaneously.
Laser light generating apparatuses that produce short-wavelength laser light by performing stepwise nonlinear wavelength conversion are known as other examples of laser light generating apparatuses in which a plurality of resonators resonate simultaneously (refer to A. Ashkin, G. D. Boyd, and J. M. Dziedzic, "Resonant optical second harmonic generation and mixing," IEEE J. Quantum Electron. QE-2, pp. 109-124 (1966) ; W. J. Kozlovsky, C. D. Nabors, and R. L. Byer, "Efficient second-harmonic generation of a diode-laser-pumped CW Nd:YAG laser using monolithic MgO:LiNbO.sub.3 external resonant cavities," IEEE J. Quantum Electron. QE-24, pp. 913-919 (1988); and D. C. Gerstenberger, G. E. Tye and R. W. Wallace, "Efficient second-harmonic conversion of cw single-frequency Nd:YAG laser light by frequency locking to a monolithic ring frequency doubler," Opt. Lett. 16, pp. 992-994 (1991)).
A method using an external resonator is known as a method for performing nonlinear wavelength conversion efficiently (refer to Z. Y. Ou, S. F. Pereira, E. S. Polzik, and H. J. Kimble, 17, pp. 640-642 (1992)). By using resonance of an external resonator, the nonlinear wavelength conversion efficiency can greatly be increased because of confinement of fundamental-wave laser light in the laser resonator. To cause the external resonator to resonate with fundamental-wave laser light, a precision positioning element in the resonator is controlled by using an FM side band method that is similar to the one used in the injection locking method.
A fourth harmonic wave with respect to fundamental-wave laser light can be generated by further wavelength-converting, by utilizing a nonlinear optical effect, a second harmonic wave generated in the above manner by SHG (Second Harmonic Generation). In this case, the wavelength shortening can be attained efficiently if the nonlinear wavelength conversion is performed by using an external resonator as in the case of the second harmonic generation process. Also in this short-wavelength laser light generation by the stepwise nonlinear wavelength conversion, it is necessary to cause a plurality of resonators to resonate simultaneously.
It goes without saying that it is necessary to cause a plurality of resonators to resonate simultaneously also in a laser light generating apparatus in which the above-described injection locking laser and nonlinear wavelength conversion process are combined.
As described above, in the laser light generating methods for generating laser light by causing a plurality of laser resonators to resonate simultaneously and establishing a phase-locked condition, control circuits are constructed independently for the control loops for controlling the resonator lengths of the laser resonators and the frequencies of phase modulation signals in all the control loops are set different from each other.
However, where phase modulation signals of the same frequency generated in a plurality of signal oscillation circuits are used simultaneously in different control loops, there may occur a case that a modulation signal in an upstream stage is mixed into an error signal in a downstream stage to disable a normal control. Further, where phase modulation signals generated by a single signal generation circuit are used in different control loops simultaneously, there may occur a case that an error signal cannot be generated correctly in a downstream loop. Still further, there may occur a case that beats of a plurality of non-lockable frequencies occur in accordance with the number of signal oscillators and hence undesired emission increases.
Usually, the Pound-Drever method (FM side band method) is used for high-precision control (locking) of a laser resonator. In the Pound-Drever method, an error signal having a bandwidth approximately equal to the control frequency bandwidth of an actuator for controlling the resonator length is generated from a phase signal of the resonator by double frequency down-conversion of "generation of a beat signal by a heterodyne method using a phase modulator (100 THz signal.fwdarw.100 MHz signal)" and "synchronous detection (10 MHz signal.fwdarw.1 kHz signal), " whereby positional accuracy of about 100 THz (i.e., accuracy of the wavelength or lower) is secured. (The "generation of a beat signal by the heterodyne method" is also a kind of synchronous detection.)
Where a plurality of resonators are controlled by signals of the same frequency that are generated by different signal oscillators, there may occur a case that a signal caused by a phase variation in an upstream resonator is mixed into a signal in a downstream resonator to make it difficult to perform a high-precision control. Also where a plurality of resonators are controlled by phase signals generated by the same signal oscillator, there may occur a case that a temporal deviation between signal inputs to an upstream resonator and a downstream resonator causes a phase variation between the resonators to prevent correct generation of error signals as in the above example.
Further, in constructing an actual circuit, high-frequency noise components in the circuit and low-frequency noise as a beat of the high-frequency noise components may act as noise (e.g., digital noise (undesired emission) in a digital circuit) that adversely affects the circuit itself. In general, using a plurality of clock generators in the same circuit is not desirable because of generation of noise of the above kind.
Where a plurality of different signal oscillation circuits are used, such circuit parts as a frequency oscillator and a frequency divider and an amplifier for the frequency oscillator are necessary for each signal oscillation circuit and hence a large circuit board mounting area is required. In particular, a plurality of signal oscillators are needed to control a plurality of resonators simultaneously but it is difficult to implement necessary circuits on the same circuit board.
That is, in constructing a control system of a laser light generating apparatus in which a plurality of laser resonators need to be controlled simultaneously, it is difficult to implement a phase modulation signal oscillator portion on the same circuit board in a compact manner.